Electrodeposition has been widely used for coating an article with a layer of electrodeposit and for electroforming an article such as a die or electrical machining (e.g. EDM) tool electrode. In electroforming, a desired metal is electrodeposited on a mold which is typically of a metallized electrically nonconductive substance and the electrodeposit is permitted to build up to a substantial thickness for subsequent removal from the mold to serve as the desired article. The deposit receiving article i.e. mold, or the "workpiece" is often large and intricate in shape or uneven (macroscopically), necessarily presenting one or more recessed areas. It is desirable that a coating or electroform be of a uniform thickness or of a desired thickness distribution over the entire area of such an intricate or uneven contour. Moreover it is often desirable that the metal deposit be thinner in projecting areas and thicker in recessed areas; however, such requirements are generally opposed to the intrinsic tendency of electrodeposition. Thus, the electrodeposit tends to be thicker in projecting areas, e.g. on ridges or convex angular portions, and to be thinner in recessed areas. In a recess, the electrodeposit tends to concentrate on the opening corner portion thereof with very little or even practically no deposit likely to occur on the floor and the corner edge portion thereof when a conventional electrodeposition arrangement is used in which the workpiece is immersed in a static mass of the electrodepositing solution and spaced with a simple planar electrode across a wide distance therein. An electrodepositing current can then be delivered only with a low current density.
On the other hand, it is known that the electrodepositing current density can be increased when the solution furnishing the depositable metal is forced to flow or floods the region of the electrode and the workpiece. While the measure to use an electrode which is shaped complementarily with the workpiece contour and to arrange such an electrode in close juxtaposition with the workpiece surface has been found to serve to increase the density of the electrodepositing current that can be delivered, and hence to increase the rate of electrodeposition, this measure has also been found to be generally unsatisfactory to drastically improve the uniformity of electrodeposit over the contoured surface apparently because the desired uniformity of distribution of the flooding solution over the entire surface is not necessarily achieved. Furthermore, this measure is totally inapplicable when selective or localized electrodeposition is contemplated.
I have found that a desired layer of electrodeposit of substantial and uniform thickness can be obtained most effectively and efficiently by employing a simple electrode having an active electrode surface much narrower than the workpiece surface to be electrodeposited and displacing the electrode relative to the workpiece to cause the active electrode surface to sweep in a scanning manner over the workpiece surface while maintaining the small gap between the electrode and the workpiece to be flooded with the electrodepositing solution, thus permitting the high-density electrodepositing current to be maintained through the gap. In this manner, an uneven and large or intricate surface having one or more recesses can be thoroughly electrodeposited, yet in a time period substantially shortened with the conventional method. Selective or localized electrodeposition and forming a layer of electrodeposit with a controlled thickness distribution can be achieved by controlling the magnitude of the electrodepositing current and/or the rate of flow of the depositing solution.
While it has been found that extremely high-speed electrodeposition can thus be obtained practically on any electrodepositable surface by maintaining a dynamic flow of the electrodepositing solution, it has now been recognized also that a problem arises as a result of the achieved improvement in the depositing efficiency and performance. Thus, the electrodepositing gap becomes a source of extremely high-rate generation of pollutants to the environment and essential equipment or parts of the apparatus. Gases are electrolytically produced from the electrodepositing gap and, together with mists, become gaseous effluents rising from the flooding solution which are harmful not only to the operator but to the environment and the sensitive elements of the apparatus. Not only are these effluents detrimental or hazardous to the operators, but such effluents, if allowed to be emitted for a prolonged time period, can cause corrosion or malfunctioning of the equipment.